1. Field of the Invention
The present invention relates to a spin valve read head sensor with a specular reflector structure located between a free layer structure and a keeper layer and, more particularly, to a specular reflector structure which reflects conduction electrons toward the free layer for increasing a spin valve effect of the sensor.
2. Description of the Related Art
An exemplary high performance read head employs a spin valve sensor for sensing magnetic fields on a moving magnetic medium, such as a rotating magnetic disk or a linearly moving magnetic tape. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90.degree. to an air bearing surface (ABS) which is an exposed surface of the sensor that faces the magnetic medium. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetic moment of the free layer is free to rotate in positive and negative directions from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from a moving magnetic medium. The quiescent position is the position of the magnetic moment of the free layer when the sense current is conducted through the sensor without magnetic field signals from a rotating magnetic disk. The quiescent position of the magnetic moment of the free layer is preferably parallel to the ABS. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen to be less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in resistance of the spin valve sensor is a function of cos .theta., where .theta. is the angle between the magnetic moments of the pinned and free layers. This resistance, which changes when there are changes in scattering of conduction electrons, is referred to in the art as magnetoresistance (MR). Magnetoresistive coeffecient is dr/R where dr is the change in magnetoresistance of the spin valve sensor from minimum magnetoresistance (magnetic moments of free and pinned layers parallel) and R is the resistance of the spin valve sensor at minimum magnetoresistance. For this reason a spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor. A spin valve sensor has a significantly higher magnetoresistive (MR) coefficient than an anisotropic magnetoresistive (AMR) sensor which does not employ a pinned layer. GMR sensors simultaneously manifest both AMR and GMR effects, so that the output signal is a superposition of the AMR signal on the GMR signal.
The transfer curve (magnetoresistive coefficient dr/R or readback signal of the spin valve head versus applied signal from the magnetic disk) of a spin valve sensor is a substantially linear portion of the aforementioned function of cos .theta.. The greater this angle, the greater the resistance of the spin valve to the sense current and the greater the readback signal (voltage sensed by processing circuitry). With positive and negative magnetic fields from a rotating magnetic disk (assumed to be equal in magnitude), it is important that positive and negative changes of the magnetoresistance (MR) of the spin valve read head be equal in order that the positive and negative magnitudes of the readback signals are equal. When this occurs the bias point on the transfer curve is considered to be zero and is located midway between the maximum positive and negative readback signals. When the direction of the magnetic moment of the free layer is parallel to the ABS, and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state, the bias point is located at zero and the positive and negative readback signals will be equal when sensing positive and negative magnetic fields from the magnetic disk. The readback signals are then referred to in the art as having symmetry about the zero bias point. When the readback signals are not equal the readback signals are asymmetric.
The location of the bias point on the transfer curve is influenced by five major forces on the free layer, namely a ferromagnetic coupling field (H.sub.FC) between the pinned layer and the free layer, a demag field (H.sub.demag) from the pinned layer, demagnetization (demag) field of the free layer acting on itself, sense current fields (H.sub.SC) from all conductive layers of the spin valve except the free layer, and the superposition of the AMR signal on the GMR signal. The influence of the AMR effect on the bias point is also defined in terms of magnitude and direction.
When the sense current is conducted through the spin valve sensor, the pinning layer (if conductive), the pinned layer and the first spacer layer, which are all on one side of the free layer, impose sense current fields on the free layer that rotate the magnetic moment of the free layer toward a first direction perpendicular to the ABS. In addition, the pinned layer demagnetization field further rotates the magnetic moment of the free layer toward the first direction counteracted by a ferromagnetic coupling field H.sub.F of the pinned layer that rotates the magnetic moment of the free layer toward a second direction antiparallel to the first direction.
It is desirable to employ a ferromagnetic keeper layer on an opposite side of the free layer from the pinned layer with a nonmagnetic electrically conductive second spacer layer therebetween. The keeper layer imposes a demagnetizing field and a sense current field on the free layer that is in an opposite direction to the aforementioned first direction so as to counterbalance the pinned layer demagnetizing field and the sense current fields from the pinning layer (if conductive), the pinned layer and the first spacer layer.
Unfortunately, in any practical sensor scheme the combination of the sense current and demagnetization fields is greater than the ferromagnetic coupling field which results in read signal asymmetry. A reduced net demagnetization field on the free layer is needed to promote read signal symmetry.
In order to reduce or even eliminate the effect of the demagnetization field of the pinned layer on the bias point of the free layer, a ferromagnetic keeper layer is provided on an opposite side of the free layer from the pinned layer with a nonmagnetic electrically conductive spacer layer between the free layer and the keeper layer. With this arrangement the keeper layer provides a flux path for the demagnetization field of the pinned layer and, in turn, the pinned layer provides a flux path for the demagnetization field of the keeper layer. Consequently, the keeper and pinned layers provide a closed loop for the demagnetization fields coming from both of these layers so that the demagnetization fields are not imposed on the free layer to influence its bias point. It is important that the magnetic moment of the keeper layer be oriented antiparallel to the orientation of the magnetic moment of the pinned layer. This can be assured by directing the sense current in a proper direction through the spin valve sensor so that sense current fields urge the magnetic moment of the keeper layer to be antiparallel to the magnetic moment of the pinned layer. An additional benefit of the keeper layer is that it exerts a sense current field on the free layer that is in an opposite direction to the aforementioned first direction so as to counterbalance the sense current fields from the pinning layer, if it is conductive, the first spacer layer and the pinned layer, on the free layer. Still another benefit of the keeper layer is that it exerts a ferromagnetic coupling field on the free layer that is antiparallel to the ferromagnetic coupling field exerted on the free layer by the pinned layer. The aforementioned benefits allow the pinned layer to be made thicker so as to increase the magnetoresistive coeffecient (dr/R) of the sensor.
Efforts continue to increase the magnetoresistance coefficient (dr/R) of spin valve sensors. An increase in the magnetoresistive coefficient equates to a higher bit density (bits/square inch of the rotating magnetic disk) read by the read head. Promoting read signal symmetry of the free layer is an important factor. These kind of efforts have increased the storage capacity of computers from kilobytes to megabytes to gigabytes.